This invention relates to methods and devices for magnetic alignment of fibres dispersed in a viscous body. The invention has particular utility in its application to alignment (parallelisation) of metal fibres, notably steel fibres, in newly cast and accordingly wet concrete and other cementitious or pasty materials. For that reason, the invention will be described with this application taken as an illustrative example.
It is known to reinforce concrete by adding steel fibres to the viscous concrete before it is cast. Usually, the fibres have a length of 2.5 to 8 cm and a diameter in the range of 0.5 to 1 mm and thus are relatively rigid. During the mixing of the fibres and the concrete, the fibres are dispersed in the concrete and orientated randomly in three dimensions so that the cast and hardened concrete body will be reinforced in three dimensions.
Many, or even most, concrete structures are only stressed in one or two dimensions, however, so that reinforcement in one or two dimensions would be adequate. This is so in the case of concrete floor slabs and concrete road pavements, to mention only two examples.
It therefore is desirable in such concrete structures to be able to align the fibres in one or two dimensions, corresponding to the direction or directions of stress, so that the fibre reinforcement material is utilised economically. It also is desirable to be able to concentrate the fibres to a zone or zones of the concrete structure where the demand for reinforcement is the greatest.
According to a known method for one-dimensional alignment of steel fibres in slabs of wet concrete newly cast in a form, a magnetic field is directed through the newly cast, viscous concrete body in the casting form and displaced relative to the form from one end or side thereof to the other in order to apply a temporary aligning force to the individual fibres for aligning them in the direction of relative movement. To facilitate the aligning movement of the fibres under the action of the magnetic field, the concrete body is vibrated during the relative movement of the magnetic field and the concrete body.
In the known method, the magnet field is applied by means of a magnet device which is positioned outside the newly cast concrete body and straddles it and also the form in which it has been cast. Magnetic fibre alignment in this manner is impracticable in many cases, however, such as in the case of concrete bodies cast in situ. Large slabs or pavements cast on the ground are two examples of concrete bodies to which the known method is difficult to apply.
In the method and device according to the present invention as defined in the claims, magnetic alignment of magnetisable fibres dispersed in a viscous body is carried out by means of a fibre aligning member having a nonmagnetic wall. A magnetic field is directed into the viscous body through a first portion of the nonmagnetic wall while the fibre aligning member is being moved relative to the viscous body with the nonmagnetic wall in contact with it with a second portion of the nonmagnetic portion trailing the first portion. Accordingly, the fibres are temporarily subjected to the magnetic field as the first portion moves past them.
The fibre aligning member may be partially or completely immersed in the viscous body as it is moved relative to the viscous body with the first portion of the magnetic wall ahead of the second portion and thus trailed by the latter.
During the relative moment, the fibres in the vicinity of the first portion of the nonmagnetic wall are magnetically attracted towards the first portion. However, they are prevented from coming into contact with the magnetic device by the nonmagnetic wall, which forms a screen or barrier that separates the magnet device from the viscous material in which the fibres are dispersed.
The fibre aligning member therefore attracts the fibres and tends to pull them along in the direction of its movement relative to the viscous body. Because of its viscosity, the material of the viscous body prevents the fibres from moving too rapidly towards the aligning member and sticking to it. Thus, the fibre aligning member will move relative to the fibres and subject them to the magnetic force only temporarily. Since the magnetic force has a component in the direction of relative moment of the fibre aligning member and the viscous body, it tends to align the fibres in that direction as it moves past them.
Preferably, the material from which the viscous body is formed is vibrated adjacent the fibre aligning member so that the aligning movement of the fibres is facilitated.
It accordingly is possible, applying the principles of the invention, to align randomly dispersed fibres in a cementitious or other viscous or pasty material in a simple manner. At the same time, a concentration of the fibres to a plane along which the fibre aligning member is moved is achieved. This plane may be in a zone of the viscous body, which in use of the hardened concrete body will have to absorb a heavy tensile stress.